MIAMI BEACH , Florida -LRB- CNN -RRB- -- On the grainy , silent black and white video , it 's hard to tell exactly what happened the night of June 14 in Miami Beach . But one thing is certain : A tourist , Husien Shehada , can be seen falling to the ground , shot by a police officer . Shehada , 29 , later died .

Husien Shehada , at left with his brother , Samer , was shot dead by Miami Beach police on June 14 .

Four nights later , again in Miami Beach , Lawrence McCoy allegedly pistol-whipped a cab driver and led police on a chase . Police say shots were exchanged . McCoy , also 29 , was killed .

Officer Adam Tavss , a 34-year-old former history teacher with three years on the force , was involved in both police shootings , the first in Miami Beach since 2003 .

Although it is not yet clear whether Tavss fired one of the shots that killed McCoy , questions are being raised as to whether the officer was cleared for patrol duty too soon after the first shooting .

Police and the Miami-Dade County state attorney 's office are investigating the shootings . The inquiry is expected to last several more months . All the records and reports have been sealed . Watch surveillance video of the first shooting ''

The families of Shehada and McCoy are asking the Justice Department to investigate . Tavss , who is now assigned to desk duty , declined through his attorney to speak with CNN , citing the ongoing investigation .

Miami Beach Police Chief Carlos Noriega has defended both shootings .

`` It is important to note that the subjects in both cases had exhibited aggressive , violent , non-compliant and criminal behavior , '' the chief said in a statement to the media .

Noriega added that officers `` are required to make split-second decisions based on a variety of factors and can not afford to hesitate or be wrong . ''

The Miami Beach department 's standard operating procedure for use of force , a copy of which was obtained by CNN , states that any officer involved in a fatal shooting must be assigned to administrative duties for at least 72 hours . The department also mandates psychological support .

Tavss was removed from duty for 72 hours , evaluated and then cleared by Noriega to return to patrol , which is departmental policy . On his first day back out on the street , Tavss was involved in the second shooting .

Each of the nation 's 20,000 police departments sets its own policy for police involved in shootings and fatalities . Some departments keep the officers off the streets for a week or longer . In virtually all cases , psychological evaluation and counseling are mandatory . Watch CNN 's report on the shootings ''

Maria Haberfeld , a professor at John Jay College of Criminal Justice in New York and an expert on police use of force and stress management , said she had never heard of an officer being involved in two shootings within four days .

`` I think it 's a gross error of judgment for any police department to maintain a rule that allows an officer who was involved in a fatal shooting to be back on the streets four days after the incident , '' Haberfeld said .

A study by the Justice Department 's National Institute of Justice found that officers involved in fatal shootings can be affected for months afterward .

`` In the days , weeks and months that follow a shooting , officers may suffer adverse reactions such as sleep interruption , anxiety and depression , '' a report on the study said .

Officers `` experienced a range of psychological , emotional and physiological reactions that distorted time , distance , sight and sound , '' the study concluded . In some cases , the study found , officers could not recall firing their weapons .

But even experts are undecided on just how much time off the street is enough , because every incident and every police officer is different .

`` It 's hard for me to estimate whether it 's weeks or a little bit more , but certainly not days ; certainly not hours . It 's just too irresponsible toward the officer and toward the society the officer needs to serve , '' Haberfeld said .

Police officers across the country train on how to make difficult split-second decisions on the use of deadly force .

The sheriff in neighboring Broward County uses an interactive video screen that places officers into scenarios they might face while on patrol .

`` In less than half a second , your pulse may go from 60 to 160 , 170 , and your heart 's beating out of your chest , and you have to make a split-second decision , '' Broward Sheriff 's Sgt. Bill Pennypacker said .

Witnesses , police and surveillance video obtained by CNN provide the following , sometimes conflicting , accounts of what happened during the two Miami Beach shootings :

Husien Shehada and his brother , Samer , had come from Virginia to spend a long weekend with their girlfriends in Miami Beach . Early on Sunday , June 14 , they got into a scuffle with another group of men , who intervened after Samer Shehada allegedly assaulted his girlfriend .

Investigators said they believe that the two brothers were looking for revenge against those men when somebody called 911 , reporting that two men were walking down the street and that one of them might be carrying a machine gun . The callers said they could see the outline of a large gun underneath his white shirt , and he was carrying it in his left hand .

In a color security camera tape released by the Miami Beach Police Department , it appears that one of the brothers was carrying something under his shirt , holding it with his left hand .

Tavss and other officers approached the brothers in front of Twist , a popular South Beach nightclub . A grainy black and white videotape shows the brothers together and Husien Shehada falling to the ground . A police officer can then be seen approaching .

Samer Shehada said he and his brother were cooperating . `` His hands were up for a good two seconds , three seconds , '' Samer Shehada said . `` He was n't in the process of raising his hands . His hands were up . ''

One witness told CNN that when Tavss approached the brothers with his gun drawn , the brothers were belligerent . The witness , Derek Reynolds , was working security at the club that night . He said the two men cursed at police and would n't put their hands up .

`` It got intense . ... They were n't cooperating , '' he said . `` One guy reached behind him , and he got shot . ''

Law enforcement sources say a coat hanger and a bottle were found at the scene , but no gun was found . Samer Shehada said that neither he nor his brother was carrying a coat hanger , much less a gun .

After the shooting , Tavss was relieved of duty for 72 hours , per department policy .

When he returned to street patrol , he was one of several officers who responded to a 911 call reporting that that a taxi driver had been pistol-whipped and his cab stolen .

Police confronted Lawrence McCoy on a Miami Beach causeway . According to police , McCoy ran and fired at the officers . McCoy was shot dead by police . It is not yet known whether Tavss fired any shots .

John Contini , an attorney representing the families of both men shot by police , said McCoy was shot nine to 11 times .

No weapon was found on McCoy . Police found a gun several days later in Biscayne Bay , but they say they do n't know if it belonged to McCoy .

Lawrence McCoy Sr. said his son 's civil rights were violated .

`` Michael Vick was convicted and put in jail for two years for killing dogs -- for killing dogs . I want justice for my son , '' he said .

Contini said Tavss should be in jail . He also questioned the policy that allowed him back on the street so quickly .

`` In fairness to the officer , you do n't put him on the street , with a gun and a badge , to be in this situation without allowing for a period of decompression -- for the officer 's sake , to get some help , '' Contini said .

@highlight

Miami Beach police officer Adam Tavss is involved in two police shootings

@highlight

Second shooting happened on his first day back on street

@highlight

Questions are being raised as to whether he was cleared for duty too soon

@highlight

Miami Beach police chief decides when an officer is ready to return